We dive from CN and am pretty sure they go mostly to the same places. Don't waste your money on an off-site dive shop; the diving is great with the resort. Snorkeling...couldn't tell you, I went straight to diving.

We dove through CSA and I wouldn't pay to go offsite. They would likely dive the same sites anyway, and the dive staff and dive boat at CSA were excellent. Plus it's super convenient to just wander down to the beach. They only go out for 1 dive at a time (except Sat) so you can choose to go back out for dive 2, or not, or just do the afternoon dive. The service was great.
Oh, and we did a night dive. It was pretty good. We did see some of the typical night creatures.
Diving in Jamaica is what it is. It's not the greatest in the Caribbean, but it's fun and easy and added to our stay at CSA... and we did get to see dolphins while diving, which was a big highlight.

I've dove at CN, but not CSA, but they most likely cover the same sites, because they are really close to each other. The diving is surprisingly awesome and you don't need to go off site. We have dove Cozumel, Cayman, Dom Rep, Aruba, St Thomas, both Gulf and Atlantic Florida, and the Negril dive sites were easily just as good. There may not be as many sites to visit, but what they have is definitely worth it. You are not gonna get anything off-resort that you wouldn't on-resort. The biggest bummer about the Couples diving is that you can only do one tank in the morning and one tank in the afternoon, unless you also pay for a night dive. We really like knocking out 2 dives in the morning, so we can start having cocktails when we're done, but we made it work. In fact, this will be the first resort that we have ever visited twice. It's easily one of the best resorts in the Caribbean.